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SNAP Fine Guidance Art Bradley/ spacecraft System Engineering Services

SNAP Fine Guidance Art Bradley/ spacecraft System Engineering Services Landis Markley/ NASA/GSFC Nov. 16, 2001. Observing strategy An acquisition strategy is proposed to conduct field mapping and spectroscopy. Field mapping is performed with a dwell scan algorithm

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SNAP Fine Guidance Art Bradley/ spacecraft System Engineering Services

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  1. SNAP Fine Guidance Art Bradley/spacecraft System Engineering Services Landis Markley/NASA/GSFC Nov. 16, 2001 G&C

  2. Observing strategy • An acquisition strategy is proposed to conduct field mapping and spectroscopy. • Field mapping is performed with a dwell scan algorithm • Simple flight code and command I/F • Primary eigenvector • Secondary eigenvector • Number of dwell points • Jerk pulse width • Constant rate interval • Primary eigenvector jerk pulse magnitude • Secondary eigenvector jerk pulse magnitude • Dwell time • Fine guidance CCD damps drift rate during shutter closure. G&C

  3. Stellar distribution • Star tables of candidate Guide Stars provided for planning purposes only. • Two 25 deg2 fields have been compiled with centers at 16h25m +57º and 4h30m -52º • Started with HST GSC. • Supplemented with latest Yale Bright Star Catalog & removed duplicates. • Supplemented with USNO A2.0 to get beyond 18mv & removed duplicates. A2.0 stars only populated in the 4ºx4º central region. G&C

  4. For the purposes of this study, 16 sq deg regions were selected near the NEP and SEP, to determine guide star probability. • Northern region • RA from 16h17m to 16h33m • DEC from +55º to +59º • Southern region • RA from 4h22m to 4h38m • DEC from -54º to -50º G&C

  5. Stellar distribution near NEP G&C

  6. Stellar distribution near NEP G&C

  7. Stellar distribution near SEP GSC population from 15.5 to 16mv not strong, should have used A2.0 in that region. G&C

  8. Stellar distribution near SEP GSC population from 15.5 to 16mv not strong, should have used A2.0 in that region. G&C

  9. FGS Design – Area • 4 geometric arrangements were examined • 200x200 arcsec - one CCD • 400x400 arcsec - four CCD’s • 200x400 arcsec - two CCD’s column stacked • 400x200 arcsec - two CCD’s row stacked G&C

  10. FGS Design – placement G&C

  11. FGS Design – Guide Star Probability • Determined with 100 arcsec increments in Right Ascension and Declination. • NEP mv range is 5.5 to 18.2 • SEP mv range is 6.1 to 17.6 G&C

  12. FGS Design – Guide Star Probability • Metric will be reduced for near neighbor’s and exclusion areas around CCD edge resulting from attitude Star Tracker error. G&C

  13. FGS Design – Histograms G&C

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  21. FGS Design – Roll Control • CCD’s placed on opposite ends of the focal plane can provide: • optional roll control • disturbance detection • Small focal plane X-Y disturbances will be greatly magnified in the roll axis. • This indicator (even if not used for control), could assist in post facto image analysis and on-board Loss-Of-Lock detection. G&C

  22. Recommendation • 200x400 arcsec CCD’s on opposite ends of the focal plane • Redundancy • Roll control •  95% Guide Star probability after star elimination. G&C

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